The vertebrate amiloride-sensitive epithelial sodium channel (“ENaC” or “amiloride-sensitive sodium channel”) is a member of the degenerin/ENaC channel superfamily, characterized by two membrane-spanning domains, intracellular N- and C-termini, and a large extracellular loop which is a substrate for furin proteases. The channel is a heterotrimeric complex composed of three homologous subunits (alpha (α), beta (β), and gamma (γ)) encoded by three separate genes: SCNN1A (alpha), SCNN1B (beta), and SCNN1G (gamma). All three subunits are required for full channel activity. A fourth subunit (delta (δ)) encoded by SCNN1D is expressed in testes and ovaries and may be able to functionally substitute for the alpha (α) subunit in those tissues. ENaC is expressed on the apical membrane of epithelial cells, particularly in the lung, renal distal convoluted tubule, gastrointestinal (GI) tract, reproductive tract, and ocular surface epithelium in the eye. In these epithelia. ENaC channels mediate influx of extracellular sodium ions which are then actively transported from the cell by the basolateral sodium/potassium ATPase, establishing an osmotic gradient and causing epithelial luminal water to be absorbed into the interstitium. In the kidney, ENaC mediates electrolyte balance and blood pressure, and is the target of systemic small molecule diuretics such as amiloride. In the lung, airway epithelial ENaC plays a key role in the regulation of lung hydration and mucociliary clearance.
Type 1 pseudohypoaldosteronism (PHA) patients that carry loss-of-function mutations in SCNN1A, SCNN1B, or SCNN1G, produce excess airway surface liquid and have significantly higher mucociliary clearance rates. Conversely, airway epithelial ENaC activity is significantly elevated in cystic fibrosis (CF) patients of all genotypes. Enhanced ENaC activity, together with reduced cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel activity, is the primary pathogenic mechanism that underlies airway dehydration and mucociliary stasis in CF lung disease patients.
Inhaled small molecule ENaC inhibitors have shown initial promise in the treatment of CF, but their clinical development has been limited by short duration of action in the lung and on-target toxicity (hyperkalemia) associated with inhibition of renal ENaC. (See, e.g., O'Riordan et al., 27 J. Aerosol Med. & Pulmonary Drug Dev., 200-208 (2014)).
Certain RNAi agents capable of inhibiting the expression of an alpha-ENaC gene (i.e., SCNN1A) have been previously identified, such as those disclosed in, for example, U.S. Pat. No. 7,718,632. However, the sequences and modifications of the alpha-ENaC RNAi agents disclosed herein differ from those previously disclosed or known in the art. The alpha-ENaC RNAi agents disclosed herein provide for highly potent and efficient inhibition of the expression of an alpha-ENaC gene.